Process for preparing fluoropolymer

ABSTRACT

A process for preparing a fluoropolymer by the batchwise polymerization of a monomer mixture containing tetrafluoroethylene, in which, after the initiation of a reaction, the monomers are reacted under conditions such that the monomer mixture is supplemented to a polymerization system in an amount more than the amount of the monomers consumed and simultaneously the excessive unreacted monomers are discharged from the polymerization system to maintain the monomer composition in the polymerization system substantially the same as that of the composition of the monomer mixture initially charged, and the monomer mixture supplemented to the polymerization system contains a compound suppressing the autoexplosion of the monomers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing afluoropolymer. In particular, the present invention relates to a processfor safely carrying out a batchwise polymerization by making thecomposition of a monomer mixture, which are supplemented under a highpressure, outside an explosion limit.

2. Prior Art

When a fluorine-containing copolymer comprising hexafluoropropylene(HFP) is prepared, a monomer mixture in a polymerization system shouldcontain 50 to 90% by mole of HFP to obtain a copolymer containing 5 to20% by weight of HFP. In addition, usually the supplemented monomermixture should be supplied under a high pressure of at least 1 MPaG.

In general, the HFP content in the obtained copolymer is less than theHFP content in the monomer mixture present in the polymerization systembecause of the different of the polymerization reactivities of HFP andother monomer(s). This means that the other monomers are more consumedthan HFP. Thus, the HFP content in the supplemented monomer mixture ismaintained less than 30% by mole. However, the monomer mixturecontaining less than 30% by mole of HFP is in the explosion limitdepending on the process pressure. Nevertheless, the supplementedmonomer mixture should be pressurized and supplied in the polymerizationsystem. Thus, the polymerization process is always threatened with thedanger of explosion.

For example, in Examples described in JP-B-61-43364, after water and adispersant are charged in a reactor, HFP and tetrafluoroethylene (TFE)are supplied, and then a polymerization initiator is charged to initiatethe polymerization. After the initiation of the polymerization, TFE issupplied in the reactor with varying a stirring speed to maintain apolymerization pressure at 4.1 MPa.

In the conventional batchwise process for preparing a copolymer of TFEand ethylene, as described in, for example, JP-A-6-211933, thepolymerization is carried out with adding perfluorocyclobutane to themixture of TFE and ethylene so that the composition is outside theexplosion limit. To increase the productivity, it is necessary toincrease the polymerization reaction rate. When the polymerizationpressure is increased to increase the reaction rate, it is inevitable toincrease the pressure of the monomers supplied. To maintain thecondition outside the explosion limit under such a high pressure, thecontent of perfluorocyclobutane in the monomer mixture should beincreased. However, if the content of perfluorocyclobutane is increased,the amount of perfluorocyclobutane in the polymerization reactorincreases and thus the space volume utilized by the polymerization inthe reactor decreases. Therefore, the yield of the polymer per unitvolume of the reactor decreases.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a process foreffectively preparing a fluoropolymer by a batchwise polymerizationwhile supplying a monomer mixture comprising TFE safely.

According to the present invention, there is provided a process forpreparing a fluoropolymer comprising batchwise polymerizing a monomermixture containing tetrafluoroethylene and optionally at least onecomonomer selected from the group consisting of hexafluoropropylene,vinylidene fluoride, trifluorochloroethylene, ethylene andperfluoroalkyl vinyl ethers, wherein after the initiation of a reaction,the monomers are reacted under conditions such that the monomer mixtureis supplemented to a polymerization system in an amount more than theamount of the monomers consumed and simultaneously the excessiveunreacted monomers are discharged from the polymerization system tomaintain the monomer composition in the polymerization systemsubstantially the same as that of the composition of the monomer mixtureinitially charged, and the monomer mixture supplemented to thepolymerization system contains a compound suppressing the autoexplosionof the monomers.

Herein, the term “autoexplosion” is intended to mean the abrupt increaseof a temperature and a pressure caused by the abrupt self-decompositionof TFE. The term “explosion limit” is intended to mean marginalconditions (e.g. the monomer composition, pressure, etc.) where themonomer gas explodes when an energy of 1 to 2 J is applied to themonomer gas sealed in a closed vessel. When the monomer mixture explodesby the application of an energy of 1 to 2 J, the monomer mixture iswithin the explosion limit.

DETAILED DESCRIPTION OF THE INVENTION

The fluorine-containing polymer or copolymer prepared by the process ofthe present invention is a polymer comprising TFE. In the case of thecopolymer, examples of the comonomer include hexafluoropropylene (HFP),vinylidene fluoride (VdF), trifluorochloroethylene (FCl), ethylene (Et)and perfluoroalkyl vinyl ethers (PAVE) (e.g. perfluoromethyl.vinyl ether(PMVE), perfluoroethyl vinyl ether (PEVE), perfluoropropyl vinyl ether(PPVE), etc.), and the like.

According to the present invention, the monomer or the monomer mixturecontains a compound suppressing the autoexplosion of the monomer(s).Examples of such a compound include HFP which is a monomer and alsofunctions as a compound suppressing the autoexplosion of the monomer(s);a compound which is used as a solvent for the polymerization and doesnot substantially interfere with the polymerization, for example,perfluorocyclobutane (C318), etc.; and a compound which is used as adiluent of the monomer(s) and does not substantially interfere with thepolymerization, for example, perfluoromethane, perfluoroethane,perfluoropropane, perfluorobutane, etc. These compounds may be usedindependently or as a mixture of two or more of them.

In the process of the present invention, TFE and at least one monomerselected from the group consisting of hexafluoropropylene, vinylidenefluoride, trifluorochloroethylene, ethylene and perfluoroalkyl vinylethers is batchwise polymerized.

In a preferred embodiment of the present invention, the monomerssupplemented to the polymerization system are tetrafluoroethylene andethylene, a compound suppressing the autoexplosion of the monomers isperfluorocyclobutane, and the content of perfluorocyclobutane is atleast 5% by mole, or the monomer supplemented to the polymerizationsystem is tetrafluoroethylene, and a compound suppressing theautoexplosion of the monomer is at least one compound selected from thegroup consisting of perfluoromethane, perfluoroethane, perfluoropropaneand perfluorobutane.

In a more preferred embodiment, the monomer mixture supplemented to thepolymerization system comprises tetrafluoroethylene, hexafluoropropyleneand optionally ethylene and/or vinylidene fluoride, andhexafluoropropylene is used as a compound suppressing the autoexplosionof the monomer. In this case, the content of hexafluoropropylene is atleast 35% by mole, while the fluoropolymer obtained contains less than35% by mole of hexafluoropropylene.

In a particularly preferred embodiment, the monomer mixture supplementedcontains tetrafluoroethylene and hexafluoropropylene;tetrafluoroethylene, hexafluoropropylene and ethylene, ortetrafluoroethylene, hexafluoropropylene and vinylidene fluoride.

The specific examples of the polymers and their compositions, thecharged monomers and their compositions, and the compound suppressingthe autoexplosion of the monomers (explosion-suppressing compound) arelisted below.

TABLE 1 Explosion- Polymer Charged monomer suppressing (composition:molar ratio) (molar ratio) compound TFE-HFP-PAVE copolymer TFE/HFP/PAVE= HFP (TFE/HFP/PAVE = 10-65/90-35/0-10 65-95/5-30/0-2) TFE-HFP-VdFcopolymer TFE/HFP/VdF = HFP (TFE/HFP/VdF = 20-50/30-60/20-5040-50/5-15/40-50) TFE-HFP-Et copolymer TFE/HFP/Et = HFP (TFE/HFP/Et =30-40/50-60/5-20 30-50/5-25/33-55) TFE-Et copolymer TFE/Et/C318 = C318(TFE/Et = 40-60/30-50/5-25 40-60/40-60) TFE-PMVE copolymer TFE/PMVE/CF₄= CF₄ (TFE/PMVE = 5-90/10-90/5-20 97-50/3-50) TFE-PAVE copolymerTFE/PAVE/CF₄ = CF₄ (TFE/PMVE = 70-95/1-10/5-30 95-9913-50) PTFE(homopolymer) TFE/CF₄ = CF₄ 70-95/5-30

The amount of the compound suppressing the autoexplosion of themonomer(s) can be suitably determined by making reference to the abovecompositions in Table 1 so that the composition of the monomer mixturecharged is outside the explosion limit, since it depends on the kinds ofthe monomers, the polymerization pressure and temperature, and the kindof the compound suppressing the autoexplosion of the monomer(s).

In the course of the polymerization, the compound suppressing theautoexplosion of the monomer(s) should be removed from the reactor sothat it is not accumulated in the reactor. Thus, the excessive amount ofthe monomer(s) should be supplied in the reactor to maintain thepressure in the reactor at the determined level.

The mixture of the monomer(s) supplemented and the compound suppressingthe autoexplosion of the monomer(s) is supplied in the reactor under apressure of preferably at least 1 MPaG, more preferably at least 2 MPaGand not exceeding the pressure resistance limit of the reactor.

The mixture of the unreacted monomer(s) removed from the polymerizationsystem maybe recycled to the polymerization system, if desired.

In such a case, the total pressure of the unreacted monomer(s) removedfrom the polymerization system and the compound suppressing theautoexplosion of the monomer(s) is once reduced to a pressure lower thanthe total pressure of the monomer(s) supplemented to the polymerizationsystem, and then the monomer necessary to restore the composition of themonomer(s) to be supplemented is supplemented, and the mixture ispressurized and recycled to the polymerization system.

In particular, it is preferable that the total pressure of the unreactedmonomer(s) removed from the polymerization system and the compoundsuppressing the autoexplosion of the monomer(s) is reduced to a pressurelower than the pressure at which the monomer(s) to be supplemented hasthe autoexplosive property.

EXAMPLES

The present invention will be illustrated by the following Examples,which do not limit the scope of the invention in any way.

Example 1

A vertical type stainless steel reactor (with a volume of 3 liters)equipped with an agitator was evacuated. Then, degassed distilled water(1.8 kg) and a 10 wt. % aqueous solution of a fluorine-containingsurfactant (C₇F₁₅COONH₄) (26.5 g) as an emulsion-stabilizer were chargedin the reactor. Furthermore, a TFE/HFP monomer mixture (TFE/HFP=30/70 bymole) was charged, and the temperature of the reactor was graduallyraised while stirring. When the temperature reached 95° C., the pressurein the reactor rose to 4.2 MPaG. Thereafter, the pressure in the reactorwas controlled at 4.2 MPaG with a back-pressure regulator, and a TFE/HFPmonomer mixture (TFE/HFP=30/70 by mole) was charged in the reactor. Atthe same time, a 10 wt. % aqueous solution of ammonium persulfate (15 g)was added to initiate the polymerization reaction. Just after theinitiation of the reaction, the polymerization was continued withcharging the TFE/HFP monomer mixture (TFE/HFP=30/70 by mole) at a rateof 30 mole/hr. which was larger than the amount of the monomer mixtureconsumed until the completion of the reaction while the 10 wt. % aqueoussolution of ammonium persulfate was continuously added at a rate of 10mg/min. and the pressure in the reactor was maintained at 4.2 MPaG withthe back-pressure regulator.

After 90 minutes from the initiation of the reaction, the supply of themonomers was stopped, and the unreacted monomer mixture was removed fromthe reactor. Then, the dispersion was recovered, and nitric acid wasadded to the dispersion to coagulate the polymer. Thus,. the whitepolymer powder was obtained. The weight of the polymer powder was 445 gafter drying.

The molar ratio of TFE/HFP in the copolymer obtained was 91/9 (by mole),and MFR was 19.2 g/10 min. The MFR was measured at 372° C. according toASTM D2116.

Example 2

A vertical type glass-lined reactor (with a volume of 4 liters) equippedwith an agitator was evacuated. Then, degassed distilled water (1 kg)was charged and then a TFE/HFP/ethylene monomer mixture(TFE/HFP/ethylene=30/63/7 by mole) was charged. The temperature of thereactor was gradually raised while stirring. When the temperaturereached 55° C., the pressure in the reactor rose to 2.4 MPaG.Thereafter, the pressure in the reactor was controlled at 2.4 MPaG witha back-pressure regulator, and the TFE/HFP/ethylene monomer mixture(TFE/HFP/ethylene=30/63/7 by mole) was charged in the reactor. At thesame time, n-propyl peroxidicarbonate (NPP) (manufactured by NOFCorporation) (3 g), which was diluted to 50% by weight with methanol,was added to initiate the polymerization reaction. Until the completionof the reaction, the polymerization was continued with charging theTFE/HFP/ethylene monomer mixture (TFE/HFP/ethylene=30/63/7 by mole) inan amount which was larger than the amount of the monomer mixtureconsumed while the pressure in the reactor was maintained at 2.4 MPaGwith the back-pressure regulator.

After 60 minutes from the initiation of the reaction, the supply of themonomer mixture was stopped, and the unreacted monomer mixture wasremoved from the reactor. Then, the dispersion was recovered, and nitricacid was added to the dispersion to coagulate the polymer. Thus thewhite polymer powder was obtained. The weight of the polymer powder wasabout 100 g after drying.

The molar ratio of TFE/HFP/ethylene in the copolymer obtained was37/18/45 (by mole), and MFR was 5 g/10 min.

Example 3

A vertical type glass-lined reactor (with a volume of 3 liters) equippedwith an agitator was evacuated. Then, degassed distilled water (1.8 kg)and a 10 wt. % aqueous solution of a fluorine-containing surfactant(C₇F₁₅COONH₄) (26.5 g) as an emulsion-stabilizer were charged in thereactor. Furthermore, a TFE/HFP/monomer mixture (TFE/HFP=30/70 by mole)was charged. The temperature of the reactor was gradually raised whilestirring. When the temperature reached 95° C., the pressure in thereactor rose to 4.2 MPaG. Thereafter, the pressure in the reactor wascontrolled at 4.2 MPaG with a back-pressure regulator, and the TFE/HFPmonomer mixture (TFE/HFP=30/70 by mole) was charged in the reactor. Atthe same time, a 10 wt. % aqueous solution of ammonium persulfate (15 g)was added to initiate the polymerization reaction. Just after theinitiation of the reaction, the polymerization was continued withcharging the TFE/HFP monomer mixture (TFE/HFP=30/70 by mole) in anamount larger than the amount of the monomer mixture consumed until thecompletion of the reaction while the 10 wt. % aqueous solution ofammonium persulfate was continuously added at a rate of 10 mg/min. andthe pressure in the reactor was maintained at 4.2 MPaG with theback-pressure regulator.

The pressure of the unreacted monomer mixture recovered from theback-pressure regulator was reduced to 0.7 MPaG. To this monomermixture, a TFE monomer at 0.8 MPaG was added so that the TFE/HFP molarratio of the monomer mixture reached 30/70. Then, the monomer mixtureobtained (TFE/HFP=30/70 by mole) was pressurized to 4.2 MPaG andrecycled to the reactor.

After 92 minutes from the initiation of the reaction, the supply of themonomer mixture was stopped, and the unreacted monomer mixture wasremoved from the reactor. Then, the dispersion was recovered, and nitricacid was added to the dispersion to coagulate the polymer. Thus, thewhite polymer powder was obtained. The weight of the polymer powder wasabout 450 g after drying.

The molar ratio of TFE/HFP in the copolymer obtained was 91/9 (by mole),and MFR was 20.5 g/10 min.

What is claimed is:
 1. A process for preparing a fluoropolymercomprising: providing an initial monomer mixture containing atetrafluoroethylene monomer and optionally at least one comonomerselected from the group consisting of hexafluoropropylene, vinylidenefluoride, trifluorochloroethylene, ethylene and perfluoroalkyl vinylethers; initiating a batchwise polymerization of the monomer mixture ina polymerization system; supplementing said monomer mixture during saidpolymerization with added monomer mixture containing afluorine-containing compound to suppress autoexplosion of the monomermixture, wherein said monomer mixture is added in an amount greater thanthe amount of the monomers consumed and simultaneously removing excessunreacted monomers from the polymerization system to maintain a monomercomposition in the polymerization system substantially the same as thatof the composition of the monomer mixture initially charged.
 2. Theprocess according to claim 1, wherein said compound suppressing theautoexplosion of the monomer mixture is at least one fluorine-containingcompound selected from the group consisting of hexafluoropropylene,perfluorocyclobutane, perfluoromethane, perfluoroethane,perfluoropropane and perfluorobutane.
 3. The process according to claim1, wherein the total pressure of the monomer mixture added to thepolymerization system and said fluorine-containing compound suppressingthe autoexplosion of the monomer mixture is at least 1 MpaG.
 4. Theprocess according to claim 1, wherein the total pressure of saidunreacted monomers removed from the polymerization system and saidfluorine-containing compound suppressing the autoexplosion of themonomer mixture is reduced to a pressure lower than the total pressureof the monomer mixture added to the polymerization system, and then anamount of monomer mixture necessary to restore the composition of themonomer mixture is added, and the monomer mixture is pressurized andrecycled to the polymerization system.
 5. The process according to claim4, wherein the total pressure of said unreacted monomers removed fromthe polymerization system and said fluorine-containing compoundsuppressing the autoexplosion of the monomer mixture is reduced to apressure lower than a pressure at which the monomer mixture to be addedexhibits autoexplosive properties.
 6. The process according to claim 1,wherein said monomer mixture added to the polymerization systemcomprises at least two monomers selected from the group consisting oftetrafluoroethylene, hexafluoropropylene, ethylene and vinylidenefluoride provided that tetrafluoroethylene and hexafluoropropylene areeach present, said fluorine-containing compound suppressing theautoexplosion of the monomer mixture is hexafluoropropylene, the amountof hexafluoropropylene present is at least 35 mole %, and saidfluoropolymer obtained contains less than 35 mole % ofhexafluoropropylene.
 7. The process according to claim 6, wherein saidmonomer mixture added to the polymerization system comprisestetrafluoroethylene and hexafluoropropylene.
 8. The process according toclaim 6, wherein said monomer mixture added to the polymerization systemcomprises tetrafluoroethylene, hexafluoropropylene and ethylene.
 9. Theprocess according to claim 6, wherein said monomer mixture added to thepolymerization system comprises tetrafluoroethylene, hexafluoropropyleneand vinylidene fluoride.
 10. The process according to claim 1, whereinsaid monomer mixture added to the polymerization system comprisestetrafluoroethylene and ethylene, said compound suppressing theautoexplosion of the monomers is perfluorocyclobutane, and the contentof perfluorocyclobutane is at least 5 mole %.
 11. The process accordingto claim 1, wherein said monomer mixture added to the polymerizationsystem comprises tetrafluoroethylene, and said compound suppressing theautoexplosion of the monomers is at least one compound selected from thegroup consisting of perfluoromethane, perfluoroethane, perfluoropropaneand perfluorobutane.